Telephone hybrid
Encyclopedia
Telephone hybrids are an essential functional component of the Public Switched Telephone Network
(PSTN). The term also describes the piece of equipment used in broadcast facilities to enable the airing of telephone callers.
The need for hybrids comes from the nature of analog plain old telephone service
home or small business telephone lines, where the two audio directions are combined on a single two-wire pair. Within the telephone network, switching and transmission are almost always four-wire circuit
s with the two signals being separated. Hybrids perform the necessary conversion. In older analog networks, this was required so that repeater amplifiers could be inserted in long-distance links. In today’s digital systems, each speech direction must be processed and transported independently.
The line card
s in a telephone central office switch that are interfaced to analog lines include hybrids that adapt the four-wire network to the two-wire circuits that connect most subscribers.
The search for better telephone hybrids and echo canceler
s (a related technology) was an important motive for the development of DSP (digital signal processing
) algorithms and hardware at Bell Labs
, NEC
, and other sites.
. The send signal is applied to both the telephone line and a ‘balancing network’ that is designed to have the same impedance as the line. The receive signal is derived by subtracting the two, thus canceling the send audio. Early hybrids were made with transformers configured as hybrid coil
s that had an extra winding that could be connected out of phase. The name ‘hybrid’ comes from these special mixed-winding transformers.
An effective hybrid would have high trans-hybrid loss, which means that relatively little of the send audio would appear on the receive port. Too much leakage can cause echoes when there is a delay in the transmission path, as there is with satellite, mobile phone, and VoIP links. This is a result of a talker’s voice traversing to the far-end hybrid and returning to his own receiver with insufficient attenuation. ITU-T
Recommendation G.131 describes the relationship of echo delay vs. amplitude to listener annoyance. At 100ms, 45dB return loss is required for less than 1% of test subjects to express dissatisfaction.
Good cancellation depends upon the balancing network having a frequency-vs.-impedance characteristic that accurately matches the line. Since telephone line impedances vary depending upon many factors and the relationship is not always smooth, analog hybrids are able to achieve only a few dB of guaranteed isolation. For this reason, modern hybrids use digital signal processing to implement an adaptive least mean squares filter
that automatically detects the line’s impedance across the voice frequency range and adjusts to it. These may reach greater than 30dB trans-hybrid loss, measured with white noise as the send signal.
DSP hybrids are also called ‘line echo cancellers’ (LECs). (The phrase ‘echo canceller’ in this context is misleading since there is no cancellation of echo per se but rather of leakage from the analog line interface with very short time delay more accurately characterized as phase-shift. However, any uncancelled hybrid leakage will cause echo when the associated transmission path has delay, so the effect on the system is reduction of echo.)
Hybrids and cancellers are sometimes combined with echo suppressor
s. These work on the assumption that usually only one of the two parties to a conversation is speaking at a given time. The suppressor switches a loss into the inactive speech path, thus enhancing the echo-cancelling effect of the hybrid at the expense of simultaneous two-way conversation.
Despite being inherently four-wire, VoIP systems require hybrids when they interface to two-wire lines. A VoIP-to-Telco gateway used to interface a VoIP PBX (private branch exchange) to analog lines would contain hybrids to perform the required conversion. End-end VoIP needs no hybrids unless adaptation to a two-wire line is required.
, and a Telephone Hybrid is the device that packages all the functions needed to connect telephone lines to studio audio systems, providing electrical and physical interface between the Telco lines and studio equipment These devices often include processing in addition to the hybrid function, such as dynamics control, filtering, and equalization. Some have dynamic EQ that adjusts parameters automatically to maintain spectral consistency from widely varying source audio. Some incorporate acoustic echo cancellation to allow setups with acoustic paths between loudspeakers carrying phone audio and microphones feeding the phone lines.
In studio application, a hybrid needs particularly good send-to-receive isolation. When too much of the host audio appears at the hybrid’s output, there will be a number of defects. Distortion of the host's voice can result from the telephone line’s changing the phase of the send audio before it returns, with varying shifts at different frequencies. The original and leakage audio are mixed at the console and combine in and out of phase at the various frequencies. When this occurs, the announcer sounds either hollow or tinny as the phase cancellation affects some frequencies more than others. Audio feedback
can result from the acoustic coupling created when callers must be heard on a loudspeaker. When lines are conferenced and the gain around the loop of the multiple hybrids is greater than unity, feedback ‘singing’ will be audible. If the leakage is very high, operators will not be able to control the relative levels of the host audio and the caller since the console telephone fader will affect both signals.
The digital signal processing technology used in modern hybrids addresses the isolation requirement and implements ancillary functions.
ISDN and VoIP Telco connections theoretically have no need for hybrids. However, calls that have ISDN or VoIP on one end usually terminate to an analog line at the other, and so there is a significant source of echo from both the Telco hybrid on the line card and the phone itself. Acoustic coupling, when the microphone picks up the output of the earpiece, is another potential source of echo. Electrical pickup between analog circuits (crosstalk) is yet another. Even low echo levels can be audible when there is a long delay, as is usually the case with VoIP.
Telephone hybrids intended for studio application are usually rack-mount units that have RJ-style connectors for the telephone line and either balanced analog or AES3 audio inputs/outputs on XLR connector
s for the studio equipment connection. One, two, or more hybrids might be packaged within a unit. There are variations to accommodate either POTS, ISDN, or VoIP telephone lines. In addition to the audio functions, hybrids can include extra capabilities such as auto-answer/disconnect, DTMF detection/generation, and Caller ID
detection.
Note that telephone hybrids must be fed from the mixing console with mix-minus
to avoid feedback.
Some inexpensive adaptors connect between a telephone and its handset, with a button to activate either the handset or the adaptor. These are simple passive devices that pass the audio from the telephone without processing.
At the other end of the spectrum are systems that can handle multiple lines and connect to a computer so that a producer
can follow who is on which line, communicate with the host, and manage the order of taking calls.
Telephone hybrids are also used to interface production intercom systems to the PSTN.
In British English, broadcast telephone hybrids are 'telephone balance units' (TBUs)
. POTS codec
s, ISDN codecs, and IP codecs are used for remote broadcast
s when it is possible to have specialized equipment at both ends of the connection. POTS codecs work with analog lines, ISDN codecs with ISDN lines, and IP codecs with any link that can provide IP connectivity. These employ audio data compression and decompression at each end. Audio bandwidths up to 20kHz can be achieved this way. Compression is often via MPEG MP3
or AAC
.
Autopatch
es are telephone hybrids used by Amateur Radio
operators to interface their radio equipment with telephone lines.
Public switched telephone network
The public switched telephone network is the network of the world's public circuit-switched telephone networks. It consists of telephone lines, fiber optic cables, microwave transmission links, cellular networks, communications satellites, and undersea telephone cables, all inter-connected by...
(PSTN). The term also describes the piece of equipment used in broadcast facilities to enable the airing of telephone callers.
The need for hybrids comes from the nature of analog plain old telephone service
Plain old telephone service
Plain old telephone service is the voice-grade telephone service that remains the basic form of residential and small business service connection to the telephone network in many parts of the world....
home or small business telephone lines, where the two audio directions are combined on a single two-wire pair. Within the telephone network, switching and transmission are almost always four-wire circuit
Four-wire circuit
In telecommunication, a four-wire circuit is a two-way circuit using two paths so arranged that the respective signals are transmitted in one direction only by one path and in the other direction by the other path...
s with the two signals being separated. Hybrids perform the necessary conversion. In older analog networks, this was required so that repeater amplifiers could be inserted in long-distance links. In today’s digital systems, each speech direction must be processed and transported independently.
The line card
Line card
A line card or Digital Line Card is a modular electronic circuit on a printed circuit board, the electronic circuits on the card interfacing the telecommunication lines coming from the subscribers to the rest of the telecommunications access network.A line card commonly interfaces the twisted pair...
s in a telephone central office switch that are interfaced to analog lines include hybrids that adapt the four-wire network to the two-wire circuits that connect most subscribers.
The search for better telephone hybrids and echo canceler
Echo cancellation
'The term echo cancellation is used in telephony to describe the process of removing echo from a voice communication in order to improve voice quality on a telephone call...
s (a related technology) was an important motive for the development of DSP (digital signal processing
Digital signal processing
Digital signal processing is concerned with the representation of discrete time signals by a sequence of numbers or symbols and the processing of these signals. Digital signal processing and analog signal processing are subfields of signal processing...
) algorithms and hardware at Bell Labs
Bell Labs
Bell Laboratories is the research and development subsidiary of the French-owned Alcatel-Lucent and previously of the American Telephone & Telegraph Company , half-owned through its Western Electric manufacturing subsidiary.Bell Laboratories operates its...
, NEC
NEC
, a Japanese multinational IT company, has its headquarters in Minato, Tokyo, Japan. NEC, part of the Sumitomo Group, provides information technology and network solutions to business enterprises, communications services providers and government....
, and other sites.
Technology
The fundamental principle is that of impedance matchingImpedance matching
In electronics, impedance matching is the practice of designing the input impedance of an electrical load to maximize the power transfer and/or minimize reflections from the load....
. The send signal is applied to both the telephone line and a ‘balancing network’ that is designed to have the same impedance as the line. The receive signal is derived by subtracting the two, thus canceling the send audio. Early hybrids were made with transformers configured as hybrid coil
Hybrid coil
A hybrid coil is a transformer that has three windings, and which is designed to be configured as a circuit having four branches, that are conjugate in pairs....
s that had an extra winding that could be connected out of phase. The name ‘hybrid’ comes from these special mixed-winding transformers.
An effective hybrid would have high trans-hybrid loss, which means that relatively little of the send audio would appear on the receive port. Too much leakage can cause echoes when there is a delay in the transmission path, as there is with satellite, mobile phone, and VoIP links. This is a result of a talker’s voice traversing to the far-end hybrid and returning to his own receiver with insufficient attenuation. ITU-T
ITU-T
The ITU Telecommunication Standardization Sector is one of the three sectors of the International Telecommunication Union ; it coordinates standards for telecommunications....
Recommendation G.131 describes the relationship of echo delay vs. amplitude to listener annoyance. At 100ms, 45dB return loss is required for less than 1% of test subjects to express dissatisfaction.
Good cancellation depends upon the balancing network having a frequency-vs.-impedance characteristic that accurately matches the line. Since telephone line impedances vary depending upon many factors and the relationship is not always smooth, analog hybrids are able to achieve only a few dB of guaranteed isolation. For this reason, modern hybrids use digital signal processing to implement an adaptive least mean squares filter
Least mean squares filter
Least mean squares algorithms are a class of adaptive filter used to mimic a desired filter by finding the filter coefficients that relate to producing the least mean squares of the error signal . It is a stochastic gradient descent method in that the filter is only adapted based on the error at...
that automatically detects the line’s impedance across the voice frequency range and adjusts to it. These may reach greater than 30dB trans-hybrid loss, measured with white noise as the send signal.
DSP hybrids are also called ‘line echo cancellers’ (LECs). (The phrase ‘echo canceller’ in this context is misleading since there is no cancellation of echo per se but rather of leakage from the analog line interface with very short time delay more accurately characterized as phase-shift. However, any uncancelled hybrid leakage will cause echo when the associated transmission path has delay, so the effect on the system is reduction of echo.)
Hybrids and cancellers are sometimes combined with echo suppressor
Echo suppressor
An echo suppressor is a telecommunications device used to reduce the echo heard on long telephone circuits, particularly circuits that traverse satellite links...
s. These work on the assumption that usually only one of the two parties to a conversation is speaking at a given time. The suppressor switches a loss into the inactive speech path, thus enhancing the echo-cancelling effect of the hybrid at the expense of simultaneous two-way conversation.
Despite being inherently four-wire, VoIP systems require hybrids when they interface to two-wire lines. A VoIP-to-Telco gateway used to interface a VoIP PBX (private branch exchange) to analog lines would contain hybrids to perform the required conversion. End-end VoIP needs no hybrids unless adaptation to a two-wire line is required.
Broadcast Telephone Hybrids
In broadcast studio facilities, the name for the functional part has come to refer to the wholePars pro toto
Pars pro toto is Latin for "a part for the whole" where the name of a portion of an object or concept represents the entire object or context....
, and a Telephone Hybrid is the device that packages all the functions needed to connect telephone lines to studio audio systems, providing electrical and physical interface between the Telco lines and studio equipment These devices often include processing in addition to the hybrid function, such as dynamics control, filtering, and equalization. Some have dynamic EQ that adjusts parameters automatically to maintain spectral consistency from widely varying source audio. Some incorporate acoustic echo cancellation to allow setups with acoustic paths between loudspeakers carrying phone audio and microphones feeding the phone lines.
In studio application, a hybrid needs particularly good send-to-receive isolation. When too much of the host audio appears at the hybrid’s output, there will be a number of defects. Distortion of the host's voice can result from the telephone line’s changing the phase of the send audio before it returns, with varying shifts at different frequencies. The original and leakage audio are mixed at the console and combine in and out of phase at the various frequencies. When this occurs, the announcer sounds either hollow or tinny as the phase cancellation affects some frequencies more than others. Audio feedback
Audio feedback
Audio feedback is a special kind of positive feedback which occurs when a sound loop exists between an audio input and an audio output...
can result from the acoustic coupling created when callers must be heard on a loudspeaker. When lines are conferenced and the gain around the loop of the multiple hybrids is greater than unity, feedback ‘singing’ will be audible. If the leakage is very high, operators will not be able to control the relative levels of the host audio and the caller since the console telephone fader will affect both signals.
The digital signal processing technology used in modern hybrids addresses the isolation requirement and implements ancillary functions.
ISDN and VoIP Telco connections theoretically have no need for hybrids. However, calls that have ISDN or VoIP on one end usually terminate to an analog line at the other, and so there is a significant source of echo from both the Telco hybrid on the line card and the phone itself. Acoustic coupling, when the microphone picks up the output of the earpiece, is another potential source of echo. Electrical pickup between analog circuits (crosstalk) is yet another. Even low echo levels can be audible when there is a long delay, as is usually the case with VoIP.
Telephone hybrids intended for studio application are usually rack-mount units that have RJ-style connectors for the telephone line and either balanced analog or AES3 audio inputs/outputs on XLR connector
XLR connector
The XLR connector is a style of electrical connector, primarily found on professional audio, video, and stage lighting equipment. The connectors are circular in design and have between 3 and 7 pins...
s for the studio equipment connection. One, two, or more hybrids might be packaged within a unit. There are variations to accommodate either POTS, ISDN, or VoIP telephone lines. In addition to the audio functions, hybrids can include extra capabilities such as auto-answer/disconnect, DTMF detection/generation, and Caller ID
Caller ID
Caller ID , also called calling line identification or calling number identification or Calling Line Identification Presentation , is a telephone service, available in analog and digital phone systems and most Voice over Internet Protocol applications, that transmits a caller's number to...
detection.
Note that telephone hybrids must be fed from the mixing console with mix-minus
Mix-minus
In audio, a mix-minus is a particular setup of a mixing console, such that the output to a certain device contains everything except the input from that device...
to avoid feedback.
Some inexpensive adaptors connect between a telephone and its handset, with a button to activate either the handset or the adaptor. These are simple passive devices that pass the audio from the telephone without processing.
At the other end of the spectrum are systems that can handle multiple lines and connect to a computer so that a producer
Radio producer
A radio producer oversees the making of a radio show. There are two main types of producer. An audio or creative producer and a content producer. Audio producers create sounds and audio specifically, content producers oversee and orchestrate a radio show or feature...
can follow who is on which line, communicate with the host, and manage the order of taking calls.
Telephone hybrids are also used to interface production intercom systems to the PSTN.
In British English, broadcast telephone hybrids are 'telephone balance units' (TBUs)
Related Equipment
Telephone hybrids are used to put calls on the air from common telephones, for example for talk radioTalk radio
Talk radio is a radio format containing discussion about topical issues. Most shows are regularly hosted by a single individual, and often feature interviews with a number of different guests. Talk radio typically includes an element of listener participation, usually by broadcasting live...
. POTS codec
POTS codec
A POTS codec is a device used in broadcast engineering to send high-fidelity digital audio over regular telephone lines. A hardware codec, implemented with digital signal processing, is used to compress the audio data enough to travel at a bitrate equivalent to a 33.6k modem...
s, ISDN codecs, and IP codecs are used for remote broadcast
Remote broadcast
In broadcast engineering, a remote broadcast is broadcasting done from a location away from a formal television studio and is considered an electronic field production . A remote pickup unit is usually used to transmit the audio and/or video back to the television station, where it joins the...
s when it is possible to have specialized equipment at both ends of the connection. POTS codecs work with analog lines, ISDN codecs with ISDN lines, and IP codecs with any link that can provide IP connectivity. These employ audio data compression and decompression at each end. Audio bandwidths up to 20kHz can be achieved this way. Compression is often via MPEG MP3
MP3
MPEG-1 or MPEG-2 Audio Layer III, more commonly referred to as MP3, is a patented digital audio encoding format using a form of lossy data compression...
or AAC
Advanced Audio Coding
Advanced Audio Coding is a standardized, lossy compression and encoding scheme for digital audio. Designed to be the successor of the MP3 format, AAC generally achieves better sound quality than MP3 at similar bit rates....
.
Autopatch
Autopatch
An autopatch, sometimes called a phone patch, is a feature of an amateur radio repeater to access an outgoing telephone connection...
es are telephone hybrids used by Amateur Radio
Amateur radio
Amateur radio is the use of designated radio frequency spectrum for purposes of private recreation, non-commercial exchange of messages, wireless experimentation, self-training, and emergency communication...
operators to interface their radio equipment with telephone lines.